So, for example, we couldn't set singles at $0.70 just to match Gamestop's pricing of the entire disc. Also, that math would only really work out if people wanted to buy all of the songs, which would mean people that wanted to buy single songs or albums would get them ridiculously cheap. Finally, just because retail discs have been discounted after 2 years, that doesn't mean the DLC was any cheaper to produce.
If people want singles or an album then the DLC is for them. If people want massive chunks of songs or all 3 albums, then they should probably pick up the disc.